Reinforced composite structure

ABSTRACT

One or more plies of reinforced elastomer, particularly two plies (10),(12) of rubber reinforced with twisted filament metallic cords (14) and more particularly a tire belt made up of the two plies (10),(12) is disclosed having cords (14) of 2x.30HT construction with opposed 23° angles to the direction of reinforcement of the belt.

This is a division of application Ser. No. 133,169, filed Dec. 11, 1987,which is a continuation of application Ser. No. 836,934, filed Mar. 6,1986, now abandoned.

The present application relates to a composite laminate structure ofcord reinforced elastomer and more particularly to a cord reinforcedcomposite having rubber where preferably the cord is metallic. Even moreparticularly, the structure is for tires and preferably a tire beltwherein at least one of two plies in the belt has the cords thereinbiased with respect to the direction of rotation of the tire.

Reinforced elastomeric articles are well known in the art for examplefor conveyor or like type belts, tires etc., with cords of textileand/or fine steel wire, particularly belts for pneumatic tires with upto four plies with the cord reinforcement between adjacent plies beingopposingly biased with respect to the direction of movement of the tirewhere it is desired to reinforce in the lateral direction in addition tothe direction of rotation of the tire. Further, cords made of multitwisted filaments of fine wire with two or more filaments in a singlestrand construction having a wrap filament therearound to reinforce theabove structure have also long been known. More recently multi strandcords such as 2+7x.22+1 have been found necessary to meet the higherdemand of fatigue life for composites in tire belts but are moreexpensive to make. Most recently, there has been use of single strandcords of multi filaments which are not twisted about each other butrather twisted altogether as a bundle or bunch to simplify the cordconstruction. Higher fatigue life requirements for composites in tireshave resulted in cords with smaller filament diameter requiring morefilaments in the cord to obtain the necessary strength.

Many problems have arisen particularly with respect to bonding a plywith an opposing angle of reinforcement to an adjacent ply to form acomposite laminate. For example when the above composite laminate has aflexible rubber matrix as in a tire belt and is loaded by a uniformtensile stress resulting from both tire inflation and footprint load,the oppositely directed in-plane shear stresses in each ply result in alarge strain gradiant near each edge of the laminate as indicated inFIG. 1. Consequently under tension the cord rubber composite belt ishighly susceptible to initiation of interply shear fracture near theedge of the belt which is known as belt edge separation. Under bothstatic and cyclic loading, the belt edge separation is initiated bycracking around the individual cord ends at the cord-rubber interface ofthe plies. The load for initiation of belt edge separation constitutes athreshold level for semi-infinite fatigue life of cord-rubbercomposites. When the maximum stress during cyclic loading does notexceed the initiating load for belt edge separation, the cord rubbercomposites exhibit no damage and therefore virtually infinite fatiguelife. The load at which the composite fails in delamination measures theload, referred to below as gross load, carrying capability of thecomposite structure which is dependent upon three factors assumingadequate cord to rubber adhesion. The factors are cord modulus, theratio of cord volume to rubber volume which is often expressed as thenumber of cord ends per inch, and the angle of cord reinforcement. Asthe angle of cord reinforcement approaches the direction of rotation ofthe tire the support from the reinforcement in the lateral directionmoves toward zero. An increase in the above-mentioned two other cordrelated factors generally results in an increase of weight for the belt.Added weight means added cost and higher rolling resistance of a tire.Lighter cords with a lower modulus do not solve the problem because eventhough they have lower weight they also have a lower cord modulus whichmust be offset by increasing the ratio of cord to rubber volume. Thisincrease in cord volume is limited by the physical size of the cord andthe resulting spacing between the cords which governs the ability of therubber to penetrate between the cords for good cord to rubber adhesion.

As indicated below the present invention will be shown to havesubstantially improved the critical load for preventing belt edgeseparation without decrease of the load carrying capability of theoverall tire belt over conventional belt constructions typicallyreinforced by 2+2x.25 and 2+7x.22+1 cord constructions.

A reinforced composite structure according to the present invention ispreferably a reinforced ply of elastomer for tire belts having twolayers of cord, each cord made of two single 0.30 mm diameter filaments,said filaments made from steel with a carbon content by weight of 0.82%,each layer having 9.45 cords per centimeter laterally spaced in adirection lateral to the direction of the width of the tire at angles of23° to the direction of movement of the tire and opposed to each other.Also included is a reinforced ply of elastomer for tires having a cordof two twisted filaments, each filament of less than 0.34 mm diametermade from steel with a carbon content by weight of 0.7 to 0.9%, said plyhaving at least 8.66 cords per centimeter spaced in a direction lateralto the direction of reinforcement of the ply. Further, the reinforcedcomposite structure can comprise an elastomeric body, a plurality ofindividual twisted filament reinforcing cords of a diameter from 0.25 to0.68 mm, the cords being laterally spaced at 8.66 to 11.02 cords percentimeter in the body and the body having a modulus greater than 120MPa psi.

A composite envisioned as an invention hereof can be an elastomer body,a plurality of individual, twisted filament reinforcing cords, the cordshaving a Tabor stiffness of less than 60 grams, the ply having acritical load separation greater than 19.8 MPa with critical strain notexceeding 11.0%. Also envisioned is an elastomeric body, a plurality ofindividual, twisted filament reinforcing cords, the cords having a Taborstiffness of less than 60 grams, and a modulus greater than 190 GPa anda filament diameter of less than 0.34 mm. Finally, a composite accordingto the present invention is envisioned as an elastomeric body, aplurality of individual, twisted filament reinforcing cords, the cordfilaments having a diameter of less than 0.34 mm and a cord modulusgreater than 190 GPa, the ply having a fatigue life to withstand nodamage at 2,000,000 cycles when loaded up to 16.9 MPa and cycled at 10cycles per second.

The above composites have the advantages of a 20% increase in criticalload over a comparable composite reinforced with 2+2x.25 cord. A smallerdiameter of the cord of the reinforcement of the present inventionresults in less rubber gauge being used where a comparable thickness ofrubber is laid on each side of the reinforcing cord upon calendering.Where two filament cord is used the result is an open cord having nocore resulting in better rubber penetration. The two filament cordresults in a cost savings over cords of three filaments or more in themanufacture of the cord. A smaller diameter cord results in less weightin the reinforcement resulting in lower rolling resistance for a tirethereby reinforced. A tire belt reinforced with a two filament cordhaving filament diameters of 0.30 mm results in 1 to 2% better rollingresistance compared to the same system reinforced with 2+2x.25 cord.Similarly, the above two filament cord results in a 15% or betterplunger energy for tires over 2+2x.25 cord for comparablereinforcements. Use of a two filament cord having filament diameters of0.30 mm and made of high tensile steel results in a 16% increase in thecomposite modulus without effecting the composite strain which remainsthe same in comparison to 2+2x.25 conventional steel cord at 7.87 endsper centimeter where the end count for the two filament cord isincreased to 9.45 ends per centimeter. Finally, the above two filamentcord results in a 2 to 6% greater composite stiffness over 2+2x.25 cordreinforced composites in tire belts even though the diameter of the twofilament cord is smaller and its weight has been reduced over 2+2x.25cord.

The above advantages of the invention will become readily apparent toone skilled in the art from reading the following detailed descriptionof an embodiment of the invention when considered in the light of theaccompanying drawings in which

FIG. 1 illustrates schematically a composite of cord and rubber in aloaded and unloaded condition;

FIG. 2 illustrates a perspective of a portion of a tire having parts cutaway to illustrate a belt package having composite structures accordingto the present invention; and

FIGS. 3 and 4 are cross sections through cords in accordance with anembodiment of the present invention at points one quarter lay lengthapart from each other in the cords.

Referring to FIGS. 1 and 2 of the drawings a single ply is illustratedin an unloaded condition, shown dotted, and in a loaded condition undertension indicated by the arrows F. This ply is shown in FIG. 2 within apneumatic tire with a radial carcass and a second ply 12 forming thebelt package for the tire. Both plies are reinforced with cords 14spaced laterally to the direction of reinforcement indicated by arrows Fand preferably at an angle of 23° but with the cord angles of the twoplies 10 and 12 opposing each other. Angles of 18° to 28° forreinforcing cords are found useful.

The cords are surrounded by an elastomer preferably rubber and thecord-rubber composite structure forms the plies 10 and 12. The plies 10and 12 in turn form a laminate structure such as the belt reinforcementfor the tire illustrated in FIG. 2. It will be appreciated that otherlaminates can be formed using principals of the present invention forreinforcing other articles such as industrial belts and that a singleply of the present invention can be used with known or conventionalplies to also form new useful reinforced composite structures.

Preferably the cords 14 are comprised of two filaments of finely drawnhigh tensile steel wire twisted about each other. Preferably thefilament diameter is 0.30 mm and its tensile elastic modulus is 190 GPaor greater. The cord has a uniform lay length of 10 to 16 mm and in thepreferred embodiment 14 mm.

The cord of the preferred embodiment will be designated as 2x.30HTdesignating a two filament twisted cord having filaments of 0.30 mmdiameter of a high tensile steel wire wherein high tensile is steel madewith a carbon content by weight of 0.7 to 0.9% and preferably 0.82%.

Referring to FIGS. 3 and 4 if a cross section is taken through a ply atright angles to the cords 14 they would appear as illustrated in FIG. 3wherein d is the diameter of the cord defined by the circle inscribingthe two filaments, being 0.60 mm in the preferred embodiment.Conventionally the inscribed diameter of the cord is used to define therivet illustrated by the space designated in FIG. 3 which is a functionof the spacing of the cords laterally across the ply. While thisdefinition of the rivet, being a nominal one, is correct for the crosssection of the cord at one point as is illustrated in FIG. 4, the actualrivet h at a point a quarter a lay length down the length of the cordfrom that point illustrated in FIG. 3 is quite different. For a tightlytwisted two filament cord as illustrated in the preferred embodiment theratio of the actual to nominal rivet h,r approaches 2 to 1 between thepoint in FIG. 4 and that in FIG. 3 as nominal rivet approaches zero.This high ratio of actual to nominal rivet over the length of the twofilament cord permits closer lateral spacing of the cord in the ply overconventional and larger diameter cords such as 2+7x.22+1 and 2+2x.25.

The limit of how many cords can be placed in a rubber body to reinforceit is determined by the minimum rivet allowable for proper adhesion ofrubber to cord. For 2+2x.25 cord this limit is 7.87 ends per centimeterwhile for 2+7x.22 wire it is 6.30 ends per centimeter the end countdropping as the diameter of the wire increases as illustrated in Table 1below.

                  TABLE 1                                                         ______________________________________                                        Tensile Stress-Strain Properties                                              of `Belt` Composites                                                          Reinforce-                                                                    ment    2 + 7X.22 + 1                                                                             2 + 2X.25 2 + 2X.25                                                                             2X.30HT                                 Matrix  Comp. I     Comp. I   Comp. II                                                                              Comp. II                                ______________________________________                                        End Count                                                                             6.30        7.87      7.87    9.45                                    (ends/cm)                                                                     Nominal 0.762       0.584     0.584   0.457                                   Rivet (mm)                                                                    Cord dia-                                                                             0.838       0.686     0.686   0.610                                   meter (mm)                                                                    Cord    17          13        13      11                                      volume                                                                        content                                                                       (%)                                                                           Load                                                                          (MPa)                                                                         Critical                                                                              16.2        16.5      16.6    19.8                                    Gross   54.5        58.5      57.3    58.7                                    Strain (%)                                                                    Critical                                                                              --          10.7      11.0    10.9                                    Gross   --          20.5      21.5    20.8                                    ______________________________________                                    

Table 1 values are based on the numerical average for 4 to 7 specimenswherein the values of cord volume content are calculated from crosssectional areas of reinforced cords, neglecting the space between thefilaments. The Table's values are further based on the preferredembodiment illustrated herein of a 2 ply tire belt for radial passengertires with cords at opposing 23° angles to each other. Table 1illustrates a substantial improvement in the resistance to belt edgeseparation initiation load which is the critical load for the compositereinforced by 2x.30HT cord. For the 2x.30HT cord of the presentinvention the critical load was 19.8 MPa while for 2+2x.25 and 2+7x.22cords the same load was well below 17 MPa. Note that the smallerdiameter of the 2x.30HT cord permitted a higher end count of 9.45 endsper centimeter within the limitation of good adhesion. It was furtherfound that the end count for 2x.30HT could be raised as high as 11.02ends per centimeter without loss of required rubber to cord adhesion dueto the reduction in river.

Initially cords of two twisted filaments of diameters of up to 0.38 mmusing conventional tire cord steel were tried which had large enoughdiameters to give sufficient tensile strength of the cords but werefound to fail for lack of fatigue life of the composite. The newreinforcing cords constructed from two high tensile steel wire filamentswere found to have an increase in fatigue life of the composite inaddition to the necessary tensile strength of the cord as illustrated inTable 2 and filament diameters of less than 0.34 mm were foundsatisfactory for meeting fatigue requirements.

                  TABLE 2                                                         ______________________________________                                        Tensile Fatigue Resistance of `Belt` Composites                               in the Threshold Region                                                                 A       B          C                                                ______________________________________                                        Reinforcement                                                                             2 + 2X.25 2 + 2X.25  2X.30HT                                      Matrix      Comp. I   Comp. II   Comp. II                                     End count   7.87      7.87       9.45                                         (ends/cm)                                                                     Maximum Cyclic                                                                            16.9      16.9       16.9                                         Load (MPa)                                                                    (lb/in width)                                                                 # Cycles to 185,910   213,510    No damage at                                 failure               178,540    2,000,000 cycles                             ______________________________________                                         Note: The frequency of 10 cycles/sec is used.                            

Further for the same diameter and construction reinforcing cords made ofhigh tensile steel wire exhibited lower weight for equivalent modulusvalues compared with those made of conventional steel wire for tire cordof a lower carbon content. For example, the weight of 2+2x.25 hightensile cord is 4% lower than that of 2+2x.25 cord of conventional steelfor tire cord.

For a given wire system two twisted filaments provide the lightest andsimplest cord construction which has the flexibility of a multi filamentcord. It has been determined that the use of a single filament ofsufficient diameter to provide the necessary tensile strength for a tirebelt reinforcement lacks the flexibility to give the necessary fatiguelife required for the belt composite.

It was experimentally observed that when the filament diameter of thepreferred embodiment cord, two filament cord, exceeded 0.34 mm Taborbending stiffness of the cord departs sharply from that of conventionalwire reinforcement with a similar cord diameter.

The above results suggest that a new metallic cord constructed from twohigh tensile steel wire filaments with individual filament diameters ofless than 0.34 mm would be the preferred choice for belt reinforcementin terms of bending stiffness and weight of the cord. More particularly,2x.30HT cord becomes a direct substitute for 4x.25 or 2+2x.25conventional steel tire cord. While the 2x.30HT cord is a directsubstitute it should be noted that it has a lower weight and smallercross sectional area compared with 2x.30 conventional steel cord and2+2x.25 cord respectively (see Table 3).

                  TABLE 3                                                         ______________________________________                                        Relative Weight of Steel Cord Reinforcement                                   Construction   Relative Weight                                                ______________________________________                                        2x.30HT        88                                                             2x.32HT        90                                                             4x.25          94                                                             2 + 2x.25HT    96                                                             2 + 2x.25      100                                                            2 + 2x.28HT    100                                                            2 + 2x.28      104                                                            2 + 7x.22 + 1  100                                                            ______________________________________                                         Note: The calculation of relative weight is based on the weight of wire       reinforcment and 1 mm thick rubber insulation on the top and bottom of th     wire.                                                                    

It was also found that compared to conventional construction of similarcord diameter for example 2+2x.25 the cut ends of two filament cord suchas 2x.30HT of the present invention are found to be more likely toseparate where they are cut often referred to as flara (see Table 4).This increased tendency of filament separation at the cut ends of thecords can result in an increased constraint over a local region of therubber matrix at a belt edge.

                  TABLE 4                                                         ______________________________________                                        Flare Length (mm) at 1 min after Cutting                                      2 + 2X.25       2X.30HT                                                       (14 mm Lay Length)                                                                            (14 mm Lay Length)                                            ______________________________________                                        8         0                  17   10                                          0         5                  5    5                                           8         0                  20   3                                           8         5                  5    6                                           7         0                  14   11                                          10        5                  5    6                                           5         10                 10   7                                           2         6                  5    6                                           2         7                  3    4                                           8         7                  8    5                                           Ave       5                  Ave  8                                           ______________________________________                                    

Some of the more unexpected results of the belt package of the preferredembodiment include a greater belt stiffness of 2 to 6% over thecomparable 2+2x.25 cord reinforced belt package wherein usually asmaller diameter cord in a belt of lighter weight would give lessstiffness in a tire belt reinforcement. Initial experimental data alsoindicates that the belt of the preferred embodiment also gives betterride characteristics of the tire while at the same time improvinghandling characteristics of the tire. Ride is a quality associated withhoop stiffness and handling is associated with the belt stiffness andwhile a softer ride usually is obtained at the sacrifice of betterhandling, i.e., a stiffer belt, in the present belt package both asofter ride and improved handling were achieved.

Also it is observed that even at the preferred belt embodiment end countof 9.45 ends per centimeter in the smaller diameter cord there is agreater plunger energy achieved over a comparable 2+2x.25 cordreinforced belt which is a measure of resistance to penetration of atire by foreign objects.

While the preferred embodiment has been described herein as a two plybelt, the present invention would also apply to belts for tires havingmore than two plies as well as to other articles of multiple plies. Aspointed out about, even a single ply in accordance with the presentinvention would be useful with conventional plies and further a singleply could well be useful in tires or other articles as a single plyreinforcement. The reinforcement of this invention is not limited to theparticular embodiment illustrated nor to necessarily the preferred twofilament embodiment, rather multi filament cords of a smaller diameterusing high tensile steel filaments having the characteristic hightensile modulus coupled with the necessary flexibility for fatigue lifeand the stiffness for lateral and planar support would also fall withinthe scope of this invention.

In accordance with the provisions of the patent statutes, the principaland mode of operation of the reinforced composite structure have beenexplained and what is considered to represent its best embodiment hasbeen illustrated and described. It should, however, be understood thatthe invention may be practiced otherwise than as specificallyillustrated and described without departing from its spirit or scope.

What is claimed is:
 1. A reinforced ply of elastomer having a cord oftwo twisted filaments each filament of less than 0.34 millimetersdiameter made from steel with a carbon content by weight of 0.7 to 0.90percent, said ply having at least 8.66 cords per centimeter spaced in adirection lateral to the direction of reinforcement of the ply.
 2. Thereinforced ply defined in claim 1 wherein said cord diameter has a rangeof 0.25 to 0.68 millimeters.
 3. The reinforced ply defined in claim 1wherein said cord has a lay length of 20 to 100 times the filamentdiameter.
 4. The reinforced ply defined in claim 1 wherein the laylength of said cord is 10 to 16 millimeters.
 5. A reinforced compositestructure comprising an elastomeric body, a plurality of individual,twisted filament reinforcing cords of a diameter from 0.25 to 0.68millimeters, said cords being laterally spaced at 8.66 to 11.02 cordsper centimeter in said body and said body having a modulus greater than120 MPa.
 6. The reinforced composite structure defined in claim 5wherein each said cord has a lay length of 20 to 100 times the filamentdiameter.
 7. The reinforced composite structure defined in claim 5wherein said cords are comprised of two single filaments.
 8. Thereinforced composite structure defined in claims 5 or 7 wherein saidfilaments have a diameter of less than 0.34 millimeters.
 9. Thereinforced composite structure of claims 5 or 6 wherein the lay lengthof said cord is 14 millimeters.
 10. A reinforced composite structurecomprising an elastomeric body, a plurality of individual, twistedfilament reinforcing cords, said cords having a Tabor stiffness of lessthan 60 grams, said ply having a critical load greater than 19.8 MPa anda critical strain not exceeding 11.0 percent.
 11. The reinforcedcomposite structure defined in claim 10 wherein the critical load rangeis from 19.8 to 23.8 MPa.
 12. The reinforced composite structure definedin claim 10 wherein the critical strain range is from 9.0 to 11.0percent.
 13. A reinforced composite structure comprising an elastomericbody, a plurality of individual, twisted filament reinforcing cords,said cords having a Tabor stiffness of less than 60 grams and a modulusgreater than 190 GPa and a filament diameter less than 0.34 mm.
 14. Thereinforced composite structure defined in claim 13 wherein the cordshave a diameter of less than 0.68 mm.
 15. A reinforced compositestructure comprising an elastomeric body, a plurality of individual,twisted filament reinforcing cords, said cords having a fialmentdiameter of less than 0.34 mm and a cord modulus greater than 190 GPaand said ply capable of 2,000,000 or more cycles at a maximum load of16.9 MPa and a frequency of 10 cycles/sec.
 16. A reinforced compositestructure comprising an elastomeric body, cords of two twistedfilaments, said cords spaced parallel to each other in a plane formingthe structure with a ratio of actual to nominal rivet of 1.94 betweenpoints on the cords which are a quarter lay length apart along thelength of the cords and capable of satisfactory rubber-cord adhesion ata nominal rivet of 0.53 times the cord diameter.
 17. The reinforced plyand structure defined in claims 1, 5, 10 or 16 wherein said cords havean average flare length of 8 mm one minute after cutting the endsthereof.